Lake Tahoe is one of hundreds of lakes around the world in the midst of a warming trend. The effects of climate change are starting to complicate efforts to maintain the lake’s relatively pristine state, putting Tahoe’s sapphire blue water and its overall ecological health at risk.

Surrounded by the Sierra Nevada Mountains and stunning scenery, the lake straddles the border between California and Nevada. At 1,645 feet deep, Lake Tahoe is one of the deepest lakes in the world. It is also one of the world’s oldest, at about two million years. Water resides in the lake for about six hundred years and flows out through only one outlet: the Truckee River.

The UC Davis Tahoe Environmental Research Center (TERC) released the Tahoe: State of the Lake Report 2012 in July. According to the report, the Lake Tahoe region experienced extreme weather conditions in 2011, including one of the wettest and coldest winters on record. Given the local variability from one year to the next, researchers working on the lake benefit from an in-lake monitoring program that stretches back to 1968. This helps them put their results into the context of the long-term record.

Lake Tahoe warming at all depths

In 2006, researchers on Lake Tahoe became curious about whether they could find any climate change signal by looking at their long-term lake temperature data. “We undertook a study and saw warming of the lake at all depths,” TERC director Geoff Schladow told me.

The research team compared the results to a half dozen other lakes around the world and found that the warming rate was higher than some and lower than others. “We looked at the meteorological data and found that air temperatures had been rising for a hundred years,” he said. “That’s when it suddenly became obvious.”

The findings coincided with the year TERC produced its first annual State of the Lake report. Citizens in the Lake Tahoe region were shocked to learn that the lake was warming. Now six years later, climate change is considered a major driver for ecological changes occurring in the lake, along with urbanization and invasive species.

Extended lake stratification season a concern for water quality

The 2012 report notes that the length of time that the upper waters remain stratified has increased by almost 20 days, claiming that this is “a likely outcome of climate change.” During a typical summer the lake becomes stratified, with warmer waters on top and cooler water at depth. In the winter these layers mix, a process that refreshes the lake and keeps it healthy.

The extended stratification season on Lake Tahoe has major implications for water quality. “A longer stratification period increases the risk of losing oxygen at the bottom of the lake,” Schladow explained, “and this can release a huge, almost infinite supply of phosphorus to the lake in a process known as internal loading.” Phosphorus is the limiting nutrient in Lake Tahoe. The more there is, the more algae can grow, causing a decline in water clarity.

The length of time Lake Tahoe remains stratified has increased by almost 20 days, a likely outcome of climate change. Source: TERC.

Scientists predict that other more subtle changes may cause additional water quality problems, especially if the longer stratification season extends into the rainy season when the highest concentration of particles wash into the lake.

Another concern is the frequency of deep mixing events in the lake. The entire depth of Lake Tahoe mixes on average every four years. “If deep mixing events occur less frequently or disappear altogether, the whole nature of the lake will change,” said Schladow.

Water clarity declining in summer and improving in winter

“In 2011, summer water clarity was the second worst value on record,” the 2012 report states. Researchers think the decline in summer clarity may be related to the impacts of climate change. Lake conditions in recent years are strongly favoring the growth of a tiny diatom –algae cell called Cyclotella. Although it has always been in the lake, its numbers have grown exponentially in the last five years. In Lake Tahoe, scientists are finding that times of maximum Cyclotella concentrations coincide with the lowest water clarity measurements.

Cyclotella diatoms are a very small algae less than 4 microns in size. Photo courtesy of TERC.

While the water clarity continues to decline in summer, winter clarity has been improving for a decade. Additional data would be needed to attribute how much of this is due to improvement in efforts to control urban stormwater runoff and how much is due to changing dynamics in the lake related to climate change.

Staying the course with water quality management efforts

Several years ago, TERC led a team of researchers who conducted a major study of the Lake Tahoe watershed to determine the sources of pollution entering the lake and to establish targets for pollution reduction efforts. Known as the total maximum daily loads, or TMDLs, the targets are established to help managers meet desired goals for water clarity in the lake.

A researcher measures water clarity in Lake Tahoe using a Secchi disk. Photo by Brent Allen, TERC.

Based on that study, tiny particles were considered the main cause of declining water clarity, but with changes occurring in the lake due to warmer water temperatures, researchers are now increasingly concerned about the nutrients entering the lake attached to these particles. “The way to reduce nutrient loads is to reduce the particles coming into the lake,” explained Schladow, “and this is exactly what the TMDL study proposes.”

Recognizing that little can be done at the local or regional level about climate change, Schladow shows no signs of defeat. He believes that by continuing to focus on reducing stressors such as excessive nutrient loading, something can be done to decrease the rate at which the lake loses oxygen. “In this way, we can increase the lake’s resilience so that it can withstand what may come to pass as a result of climate change.”

Making the local-global connection

The local-global connection for lake temperature research is perhaps best exemplified on Lake Tahoe, which was used to calibrate the satellite data in a global assessment of lake temperatures. Buoys deployed for that and other joint studies with NASA’s Jet Propulsion Laboratory (JPL) continue to measure lake temperature every two minutes, 24 hours a day.

“Tahoe is unique because there are very few places where all of this data is recorded on a lake of sufficiently large size that it can be compared with satellite data,” said Simon Hook, a researcher at JPL who led the global assessment. “It is why places like Lake Tahoe are so important to understanding the trends and ecological consequences.”

Tahoe’s unique characteristics make it an ideal study site. “Because Lake Tahoe is still very clean and relatively pristine, it has a high signal to noise ratio,” explained Schladow. “We can often see things that are happening in other lakes but are just harder to tease out there.”

In a previous post on warming lakes, I mentioned that my curiosity about lake temperatures was triggered by a pleasant swim in Lake Michigan this past summer. Geoff Schladow confessed that he used to joke about the benefits of being able to swim more in a warmer Lake Tahoe. He stopped joking about it because he felt it confused people. “It undervalues all that lakes have to offer,” he said. “Swimming is a trite benefit when considering all that would happen if the lake is that much warmer.”

Lisa Borre is a lake conservationist, freelance writer and sailor based in Annapolis, MD. With her husband, she co-founded LakeNet, a world lakes network that was active from 1998 to 2008, and co-wrote a sailing guide called “The Black Sea.” She is a native of the Great Lakes region and served as coordinator of the Lake Champlain Basin Program from 1990 to 1997.

Comments

These freshwater initiative articles are so applicable to watersheds throughout the world. Thank you to NG and all of your authors for covering these timely and important topics succinctly.

Judith Morris

Lake George, NY

October 23, 2012, 12:30 pm

Our family has enjoyed being at the lake for almost four
decdes. Sadly, the lake has become more polluted with time. Although it is still a clear and relatively clean lake, it shows many signs of stress. Last winter it did not even freeze over…..a very unusual event at a lake that often would freeze to a depth of 3-4 feet in the past!

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